Numerous of the combustion processes incident to power generation, generate as well as an undesired product, effluent gases having an unacceptable NO.sub.x content. More specifically, the high temperatures incident to the operation of fuel-driven turbines, internal combustion engines and the like, results in the fixation of some oxides of nitrogen. These compounds are found in the effluent gases mainly as nitric oxide (NO) with lesser amounts of nitrogen dioxide (NO.sub.2) and only traces of other oxides. Since nitric oxide (NO) continues to oxidize to nitrogen dioxide (NO.sub.2) in the air at ordinary temperatures, there is no way to predict with accuracy the amounts of each separately in vented gases at a given time. Thus, the total amount of nitric oxide (NO) plus nitrogen dioxide (NO.sub.2) in a sample is determined and referred to as "oxides of nitrogen" (NO.sub.x).
NO.sub.x emissions from stack gases, engine exhausts etc., through atmospheric reactions, produce "smog" that stings eyes and may cause or contribute to acid rain. Other deleterious effects both to health and to structures are believed to be caused directly or indirectly by these NO.sub.x emissions. For these reasons, the content of oxides of nitrogen present in gases vented to the atmosphere has been subject to increasingly stringent limits via regulations promulgated by various state and federal agencies.
To meet the regulations for NO.sub.x emissions, a number of methods of NO.sub.x control have previously been employed or proposed. In one approach water or steam are injected into the combustion zone. This lowers the flame temperature and thereby retards the formation of NO.sub.x, since the amount of NO.sub.x formed generally increases with increasing temperatures. Water or steam injection, however, adversely affects the overall fuel efficiency of the process as energy is absorbed to vaporize the water or heat the injectable steam, which would otherwise go toward heating the power source exhaust and be ultimately converted into usable steam.
It is also known to inject ammonia to selectively reduce NO.sub.x. A process involving the injection of ammonia into the products of combustion is shown, for example, in Welty, U.S. Pat. No. 4,164,546. Examples of processes utilizing ammonia injection and a reducing catalyst are disclosed in Sakari et al., U.S. Pat. No. 4,106,286; and Haeflich, U.S. Pat. No. 4,572,110. However, selective reduction methods using ammonia injection are expensive and somewhat difficult to control. Thus, these methods have the inherent problem of requiring that the ammonia injection be carefully controlled so as not to inject too much and create a possible emission problem by emitting excess levels of ammonia. In addition the temperature necessary for the reduction of the oxides of nitrogen must be carefully controlled to yield the required reaction rates.
Apparatus modifications have also been widely used or proposed as a solution to the aforementioned NO.sub.x emission problem. These include modifications to the burner or firebox to reduce the formation of NO.sub.x. Although these methods can reduce the level of NO.sub.x, each has its own drawbacks. Combustion equipment modifications can e.g. affect performance and limit the range of operation.
A selective catalytic reduction system is presently considered by some to be the best available control technology for the reduction of NO.sub.x from the exhaust gas of a cogeneration plant and, as a consequence, is often required equipment. Currently available selective catalytic reduction systems used for the reduction of NO.sub.x employ ammonia injection into the exhaust gas stream for reaction with the NO.sub.x in the presence of a catalyst to produce nitrogen and water vapor. Such systems typically have an efficiency of 80-90 percent when the exhaust gas stream is at a temperature within a temperature range of approximately 600.degree.-700.degree. F. The NO.sub.x reduction efficiency of the system is significantly less if the temperature is outside the stated temperature range and the catalyst may be damaged at higher temperatures.
A further approach to reducing NO.sub.x levels from combustion processes, is based on combustion staging. Thus a fuel-rich primary stage may be followed by secondary air addition and completion of combustion as a later stage.
Reference may be had in this connection to McGill et al., U.S. Pat. No. 4,405,587, for which the present Applicant is a co-patentee. As disclosed therein, oxides of nitrogen can be reduced by reaction in a reducing atmosphere at temperatures in excess of 2000.degree. F., for example 2000.degree. to 3000.degree. F.
U.S. Pat. No. 4,354,821 is also of interest in disclosing a system for combusting a nitrogen-containing fuel in such a manner as to minimize NO.sub.x formation. The fuel to be combusted is directed through a series of combustion zones having beds of catalytic materials. Air is added to each of two upstream zones to provide fuel-rich conditions to thereby minimize formation of NO.sub.x precursors. In a final zone also having a bed of catalytic material, excess air is provided to complete combustion of the fuel.
In my U.S. Pat. No. 4,811,555, there is disclosed a cogeneration system wherein electrical power is generated by a gas turbine. The gaseous effluent from the turbine, together with sufficient additional fuel to produce a fuel-rich, fuel-air mixture is fed to a boiler to generate steam. Air is added to the gaseous effluent from the boiler to form a lean fuel-air mixture, and this mixture is passed over an oxidizing catalyst, with the resultant gas stream then passing to an economizer or low pressure waste heat boiler for substantial recovery of its remaining heat content. The gas, now meeting NO.sub.x emission standards, is then vented to atmosphere.
It will be appreciated that in my said U.S. Pat. No. 4,811,555, a gas turbine constitutes the primary power source. The NO.sub.x levels ultimately achieved therein are quite low, i.e. below about 50 ppmv for the final gases provided for venting. Since, however, NO.sub.x levels in the turbine exhaust are not extremely high to begin with (i.e. about 150 ppmv), the actual reduction is only moderate. Where an internal combustion engine (such as a diesel) constitutes the power source, NO.sub.x levels in the exhaust are an order of magnitude higher than in a gas turbine--a typical NO.sub.x level for such an engine being about 1500 ppmv. In this instance the exhaust stream also carries substantial particulate matter in the form of unburned carbon. It is found that with such a power source, neither the methods taught in my U.S. Pat. No. 4,811,555, or those otherwise known in the prior art are adequate or effective to economically and efficiently achieve fully acceptable NO.sub.x reduction.
In my U.S. Pat. No. 5,022,226, a cogeneration method and system is, however, provided wherein the primary power source is an internal combustion engine, and wherein the quantity of NO.sub.x in the fuel emissions to atmosphere is reduced to a completely safe and acceptable level. A cogeneration system is thus disclosed in said patent wherein fuel and oxygen are provided to an internal combustion engine connected to drive an electric generator, to thereby generate electricity. An exhaust stream is recovered from the engine at a temperature of about 500.degree. to 1000.degree. F. which includes from about 6 to 15 percent oxygen. Sufficient fuel is added to the exhaust stream to create a fuel-rich mixture, the quantity of fuel being sufficient to react with the available oxygen and reduce the NO.sub.x in the exhaust stream. The fuel-enriched stream is then provided to a thermal reactor means for reacting the fuel, NO.sub.x and available oxygen, to provide a heated oxygen-depleted stream. The oxygen-depleted stream is cooled in a heat exchanger. Prior to being passed over a catalyst bed under overall reducing conditions, conversion oxygen is added to the cooled stream. Such oxygen can be provided directly (i.e. as air), but preferably can be provided by bypassing part of the exhaust stream from the engine. The quantity of conversion oxygen is stoichiometrically in excess of the amount of NO.sub.x but less (stoichiometrically) than the amount of combustibles, in consequence of which NO.sub.x in the stream is oxidized to NO.sub.2 at the forward end of the bed, after which the NO.sub.2 is reduced in the remainder of the bed by the excess combustibles. Air is added to the resulting stream from the catalytic bed to produce a cooled stream having a stoichiometric excess of oxygen, and the stream is passed over a oxidizing catalyst bed to oxidize remaining excess combustibles. The resultant stream, vastly reduced in NO.sub.x content can then be provided for venting. The NO.sub.x content can be reduced to less than 25 ppmv--often below 15 ppmv, while CO levels are also brought to well below 50 ppmv.
The methodology of my aforementioned patents, including in U.S. Pat. No. 5,022,226, has thus shown promise as an emerging technology for the reduction of NO.sub.x from internal combustion engines and industrial boilers. In the instance of certain further types of power sources, however, notably including coal-fired fluidized bed boilers, a new consideration emerges, more specifically a need for simultaneous destruction of NO.sub.x and N.sub.2 O. The level of N.sub.2 O from coal-fired fluidized bed boilers is in the range of 50-300 ppmv, and more typically 100-200 ppmv. This level is 10-20 times that found in the post flame products of other fuel-fired equipment such as conventional boilers and fired heaters. This may be attributed to the fact that a fluidized bed boiler is operated at lower combustion temperatures (&lt;1700.degree. F.) and utilizes a fuel containing bound nitrogen. It is thought that N.sub.2 O may be formed by the partial oxidation of HCN (originating in the fuel-rich devolatilization zone in coal burning) by NO at lower temperatures. Nitrous oxide has been identified as a compound which not only acts as a greenhouse gas but also contributes to stratospheric ozone depletion. Evidence indicates that the concentration of N.sub.2 O has been increasing in the atmosphere since the onset of industrialization. Thus a commensurate interest exists in reducing or eliminating discharge of N.sub.2 O from power sources such as the mentioned fluidized bed boilers.
Current state of the art methods of NO.sub.x reduction such as catalytic and non-catalytic selective reduction using ammonia injection or "reburn" fuel/air staging have the desired results of reducing NO.sub.x but do not effectively reduce N.sub.2 O. In fact, in many cases the levels of N.sub.2 O actually increase as a result of applying these technologies. It is therefore highly desirable to provide a method of NO.sub.x control that will simultaneously reduce N.sub.2 O to very low levels.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a method and system which are effective for reducing quantities of oxides of nitrogen, including N.sub.2 O that are generated in operation of a coal-fired fluidized bed boiler, to levels which are safe and acceptable for discharge to atmosphere.
Another object of the invention is to provide a method and system of the foregoing character, which simultaneously provides for useful recovery of heat values which might otherwise be lost.